To drive hybrid or electric vehicles, electric machines are normally used in the form of polyphase machines, which are operated in combination with pulse-controlled inverters, which are often also simply called inverters. The electric machines in this context are operated selectively in a motor operating mode or a generator operating mode. In motor operation, the electric machine generates a driving torque, which, when used in a hybrid vehicle, supports an internal combustion engine in an acceleration phase, for example. In generator operation, the electric machine generates electrical energy, which is stored in an energy store such as a traction battery, for example. An intermediate circuit capacitor is provided for stabilizing the battery voltage. The operating mode and the output of the electric machine are set using the pulse-controlled inverter. The pulse-controlled inverter includes power semiconductor switches—called simply power switches in the following—such as MOSFETs (metal oxide semiconductor field-effect transistors), IGBTs (insulated gate bipolar transistors) or MCTs (MOS controlled thyristor), which are controlled via a control unit.
It is fundamentally known to control the switching elements of a pulse-controlled inverter in a pulse-width modulated manner. For this purpose, a constant period duration or constant modulation frequency is normally used for the pulse width modulation (PWM). Frequently, a symmetrical pulse width modulation is used, in which an impulse is centered within a signal period such that the duration of the pause is distributed equally to the beginning and the end of the signal period.
For specific operating points of an electric machine controlled by a pulse-controlled inverter, it may be practical, e.g. for reasons of minimizing losses and/or minimizing EMC radiation (EMC=electromagnetic compatibility), to control the switching elements of the pulse-controlled inverter at a lower switching frequency, to which the system is able to switch over while the electric machine and thus the pulse-controlled inverter are in operation. A switchover to half the switching frequency has proved to be particularly advantageous.
In the simplest case, such halving of the switching frequency may be achieved by halving the modulation frequency.